The present invention relates to telecommunications, and more particularly to carrier frequency detection for an N-ary phase modulated signal.
Wireless communication by mobile phone is shifting from analog modulation to digital modulation. For example, a transmitter digitally modulates a baseband signal onto a carrier frequency and a receiver demodulates the modulated signal to obtain the baseband signal. FIG. 1 shows a block diagram of a typical demodulator for a QPSK (Quadrature Phase Shift Keying) modulated signal. The QPSK modulated signal is provided to first and second phase detectors 12, 14 and to a carrier recovery circuit 16. The carrier recovery circuit 16 recovers the carrier frequency from the QPSK modulated signal, and the carrier frequency is provided to the first phase detector 12. The second phase detector 14 receives the carrier frequency via a quadrature phase shifter 18. The outputs of the first and second phase detectors 12, 14 are provided to respective first and second sample and decision circuits 20, 22 to demodulate In-phase and Quadrature-phase (IQ) components for the baseband signal according to a timing signal from a timing recovery circuit 24.
The carrier recovery circuit 16 typically uses a Costas loop, which is a type of phase-locked loop, to recover the carrier frequency from the QPSK modulated signal. FIG. 2 shows a typical Costas loop that recovers the carrier frequency from a QPSK modulated signal. The QPSK modulated signal is provided to multipliers 30, 32, 34, 36 for multiplication by frequency signals derived from a variable frequency oscillator 38. The frequency signals provided to the multipliers 32, 34, 36 are shifted by π/4, π/2 and 3π/4 respectively, relative to the frequency input to multiplier 30 from the variable frequency oscillator, through respective phase shifters 52, 54, 56. The outputs from the multipliers 30, 32, 34, 36 pass through respective low pass filters 40, 42, 44, 46 to remove the carrier frequency component. An output multiplier 48 multiplies the outputs from the four LPFs 40, 42, 44, 46 together to produce a signal raised to the 4th power. Since the output from the output multiplier 48 is a fourth powered one relative to the original frequency of the QPSK modulated signal, the phase modulated components are converted into phase 0, 2π, 4π and 6π components, i.e., all phase 0 components, and disappear so that the output signal depends upon a phase difference Δφ between the carrier frequency and the frequency signal from the variable frequency oscillator 38. The phase difference Δφ is provided to the variable frequency oscillator 38 via a loop filter 50. The output from the loop filter 50 controls the frequency of the variable frequency oscillator 38 to minimize the output from the output multiplier 48. If this control makes the phase difference Δφ=0, the frequency signal from the variable frequency oscillator 38 is the carrier frequency. FIGS. 1 and 2 show a demodulator for a QPSK modulated signal having four phases, but a demodulator for an N-PSK modulated signal would be similar.
Alternatively, the block diagram of FIGS. 1 and 2 may be implemented in software using a computer. In this case, the QPSK modulated signal is digitized by analog-to-digital conversion using a sampling frequency before the demodulation process. If it is not required to demodulate the QPSK modulated signal in real time, such as where signal analysis involves iterative processes, the digitized QPSK modulated signal may be stored in a storage device, such as a hard disk drive, and the stored digitized QPSK modulated signal may be read out subsequently to conduct the demodulation process shown in FIGS. 1 and 2. If the process speed is fast enough, the software demodulation process may be performed in real time.
Carrier frequency recovery using a Costas loop is disclosed in U.S. Pat. No. 6,810,097 which has a loop filter circuit that includes a feedback loop with lower noise to provide fast and accurate recovery of the carrier frequency. The modulated signal is squared, i.e., multiplied by itself. A weak point of the Costas loop is that the feedback loop easily oscillates if the feedback loop is unstable. The feedback loop disclosed in the patent uses a complex circuit to produce the fast and accurate carrier frequency recovery. These are significant problems that even the software demodulation process or the ideal operations of the functional blocks may not get around.
What is desired is carrier frequency detection in a telecommunications demodulator that recovers the carrier frequency from an N-ary modulated signal while avoiding the use of potentially unstable and complex feedback loop processes.